The present invention relates generally to digital communications networks, and more specifically to signaling message routing control.
According to CCITT Recommendations Q.711-Q. 714 for Signaling System Number 7, message transfer part (MTP) serves as a transport system for users. It transfers signaling messages between user locations and is responsible for providing physical links, providing data link control functions, and routing messages between user locations. Signaling connection control part (SCCP) is situated between the user layers and the MTP to provide additional functions to the MTP and to support both connectionless and connection-oriented network services between signaling nodes, or signaling points. More specifically, SCCP management of Signaling System Number 7 maintains the congestion status of remote signaling points with which it has a signaling relationship. SCCP management also informs local users of changes in the states of point codes (signaling node identifiers) and subsystem numbers it monitors. How network congestion is to be handled within the SCCP is not specified by the Recommendations. However, it is specified that users of the SCCP are to be notified of changes in the status of point codes and SCCP users with which the SCCP maintains a signaling relationship. It is unrealistic to expect each local application process to maintain multiple point code states since they can rely on the SCCP to determine when the availability of remote point codes and subsystem numbers change.
Since many applications which use the SCCP conceivably cannot make use of point code and subsystem number data, it is desirable that SCCP provide a minimum level of checking for its users. It is further desirable to restore connectionless traffic to recovering signaling nodes in a phased manner to prevent the signaling network from suddenly being overwhelmed by a surge of traffic in a manner which does not interfere Message Transfer Part (MTP) allowed/prohibited state and SCCP allowed/prohibited state procedures. It is desirable that the phased recovery procedure be implemented in the network layer to relieve user processes from the burden of handling what is properly a network layer function.
Furthermore, it is desirable that point code availability and congestion control checks be implemented in the SCCP control to remove the requirement that each user maintain a database containing the availability and congestion states of each network address (i.e., point code plus subsystem number) with which it communicates.